A PC board assembly generally comprises a rigid nonconducting board, also known as a substrate, a plurality of metallic conductors of predetermined length on at least one side of the substrate, and a plurality of discrete electronic components secured to the board. The PC board is typically coupled to other circuitry by means of a flexible circuit tape bonded to the board or a multi-pin connector positioned on one of its edges. The term "PC board" as used herein is intended to include printed wiring boards.
The PC board is generally fabricated as follows. First, holes are perforated in the PC board substrate such as by punching, drilling or any other suitable machining process. The leads of electrical components are then inserted through the holes either manually o by machine in a "thru-hole" mounting arrangement. In some cases, the components are surface mounted on the PC board. Where components are thru-hole mounted as well as surface mounted, the PC board is described as being of mixed technology. Following component insertion, component leads are then conductively bonded to predetermined conductors on the PC board. Conductive bonding is typically accomplished by wave soldering involving passing the lower side of the PC board from which the conductive pins of the various components extend through a molten solder bath. The molten solder bath extends the full width of the PC board and covers the entire lower surface of the PC board as the PC board is transported over the solder bath. A rosin-based or organic acid flux is generally applied to the PC board prior to soldering for improved adherence.
In surface mounted technology, a solder paste is applied to designated areas of the PC board using a screen process. The solder paste is applied at room temperature on the component, or "upper", side of the PC board on "footprints" where the components, or devices, are to be mounted. Surface mounted devices may also be glued to the other side of the PC board. A surface mounted device is placed on each of the paste covered footprints and the PC boards are conveyed through an oven, or other heating apparatus, melting the paste solder in a process termed "reflow."
The solder is typically comprised of a tin-based conductive material heated to a temperature to allow it to flow and be deposited on the PC board's lower surface. This requires the solder to be heated to relatively high temperatures and frequently subjects the PC board itself as well as its associated circuitry to excessively high temperatures. When excessively heated, electronic components may fail, electrical connections may become undone, and the PC board itself may become warped preventing the solder from being deposited uniformly over its entire lower surface. Any one of these results gives rise to PC board failure either during or after production. While detection of a failed PC board can be corrected during production, this slows down production and limits final product output. A failure following production such as after purchase is frequently even more expensive and inevitably reflects adversely on the producer.